1. Field of the Invention
The invention relates to halftone image transformation, and in particular, to a method for generating a halftone mask using a Discrete Wavelet Transform (DWT) algorithm.
2. Description of the Related Art
Halftone transformation can induce an illusion effect on an image to extend the equivalent gray level variation, so that the image may look more realistic. Such a technique is widely adapted in the image processing fields, including the printing and display fields. An original image usually has gray levels higher than a printer can present, thus halftone transformation is required to reproduce an equivalent gray level effect on the printout. For example, a printer may be equipped with only black ink, and through halftone transformation, equivalent gray level effects can be rendered. Generally, an original pixel is converted into a plurality of tiny dots distributed in a particular manner. Specifically, the gray level can be simulated by dot size variation, or by dot density variation. Different dot densities can exhibit different gray level effects, and distribution of the dots are performed by a predetermined halftone mask such as Blue Noise Mask, Void & Cluster, or a real time schematic such as Error Diffusion.
FIG. 1 shows conventional printing equipment 100, comprising essential components such as at least a processor 110, a read only memory (ROM) 120 and a mechanical module 130. The ROM 120 stores a previously designed halftone mask #MASK. When an original image #IMG is sent to the printing equipment 100, the processor 110 uses the halftone mask #MASK to convert the original image #IMG into a halftone image #HALF, and thereafter, the mechanical module 130 performs physical operations to render a printout #PRINT from the halftone image #HALF. Practically, the halftone mask can be widely adapted to various applications, in addition to the printing equipment 100. However, detailed introduction is omitted herein. According to the aforementioned example, the equivalent gray level effect in the printout #PRINT is simulated by multiple tiny dots, and the uniformity of the dot distribution determines image quality of the printout #PRINT. Therefore, it is important to design a good halftone mask #MASK that uniformly distributes the tiny dots.